I think I found the culprit, it's the display settings.
I changed it from YCbCr444 to YCbCr422 and it worked like a charm. Using it like that for 2 days and have never noticed the problem again.
I have this issue sometimes on my Sony Bravia KD49XE9005. I have a google home, but I've deactivated it because it kept interfering with my TV.
Fade out, as in get quieter? That won't be anything to do with bandwidth (it's not analog!). Do you have a Google Home nearby? I've noticed devices manipulating volume if it thinks it years a keyword.